1. Field of the Invention
The present invention relates to a solid state laser rod.
2. Description of the Related Art
The lasers mainly used for material processing are gas lasers (CO2) and solid state type lasers (Nd-YAG or Nd glass). CO2 lasers are advantageous in that they allow for greater power (approx. 20 kW are commercially available). Solid state type lasers are characterized in particular by a compact construction, a smaller obtainable focus and a shorter wavelength. Since the wavelength of the solid laser is in the range of very near IR light, glass optics can be used. The disadvantages of solid lasers are the low efficiency of merely 1.5 to 2%, the lower power (about 1000 W are commercially available at present), and the poor beam quality or focusability at higher powers.
Up to now, circular laser rods have normally been used for solid lasers. A circular cross-section of the rod, however, leads to a radial temperature gradient due to the energy supply by optical pumping. Since different temperatures within the rod result in different refraction indices at different points of the rod, a thermal lens forms in the rod. This thermal lens impairs the beam quality (increasing the beam divergence) and becomes so strong, as of a certain pumping power, that a stable resonator is no longer possible.
Various shapes have been proposed for the laser-active solid state body which differ from the usual circular rod shape (e.g. FR-A-1356934 and Soviet Journal of Quantum Electronics 1,4 Jan.-Feb. 72, 369 ff).
In these proposals, the beam is reflected several times in polygonal bodies (partly utilizing the low-loss total reflection on boundary surfaces), thereby obtaining a beam path as long as possible in the laser-active medium. The beam path length is further increased by placing a plurality of such polygonal disks one on top of the other and having the beam pass through these disks successively.
Due to the long beam path, one would thus theoretically expect lasers with high output power. But in the case of optical nonuniformity (variations in the refraction index due to thermal gradients) a longer beam path means a corresponding impairment of beam quality.
Furthermore, the proposed bodies are difficult to realize in terms of their dimensions and the working of their surfaces.
In view of the thermal problems and the resulting optical problems, a laser rod has also been developed with a rectangular cross-section, the so-called "slab laser" (U.S. Pat. No. 3,633,126; U.S. Pat. No. 4,214,216). A light beam is introduced into one end of the slab, which is preferably oriented at a Brewster angle, said light beam being reflected several times within the rod on two opposed, totally reflecting longitudinal sides of the rod before exiting from the other end. With suitable pumping geometry, this results not in a radial temperature gradient as in the rod with a circular cross-section, but in isothermal planes with more favorable heat dissipation. Due to the increased heat dissipation one can obtain a higher laser output. Due to the zigzag path of the light beam within the laser rod, the beam passes through the temperature gradients in the opposite direction. This compensates the effect of the thermal lens on one axis. On the other axis no gradient occurs due to the flat isotherms.
The slab laser therefore allows for higher pumping powers and higher output power with good beam quality, but its efficiency is lower than using a circular rod due to the pumping geometry.